Oracle pl sql Error ORA-00904 invalid identifier

Causa:The column name entered is either missing or invalid.

Action: Enter a valid column name. A valid column name must begin with a letter, be less than or equal to 30 characters, and consist of only alphanumeric characters and the special characters $, _, and #. If it contains other characters, then it must be enclosed in double quotation marks. It may not be a reserved word. example:

SQL> select col1 from dual;

select col1 from dual * ERROR at line 1: ORA-00904: invalid identifier